D’Emic et al.—Revision of the sauropod dinosaur Sonorasaurus


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the quarry preserves some channel-lag deposits containing pebbles (Ratkevitch, 1998; Scarborough, 2000), so the sedimentological context speaks to an abiological origin for these clasts. Finally, point (5) is circular. Further evidence is required to substantiate the presence of gastroliths in Sonorasaurus.


Cladistic analysis


Figure 25. Bone histology of the humerus of Sonorasaurus thompsoni (ASDM 500) from the mid-Cretaceous Turney Ranch Formation of Arizona, USA: (1, 2, 3) at right indicate detail of corresponding regions of section at left. Arrowheads denote lines of arrested growth; brackets and arrowheads denote multiple lines of arrested growth deposited within one year (i.e., double LAGs). Abbreviations: po = primary osteon; so = secondary osteon.


‘Gastroliths’.—Thayer and Ratkevitch (1995) and Ratkevitch (1998) reported the presence of several gastroliths from the holotype quarry of Sonorasaurus thompsoni. Thayer and Ratkevitch (1995:74) gave five lines of evidence to support their identification of gastroliths: “(1) highly polished; (2) exotic types of rock for the region; (3) high in silica; (4) out of sedimentological context; 5) in formations known to contain fossils for gastroliths—using animals such as ornithopods and sauropods.” However, these lines of evidence are problematic, and the purported presence of gastroliths in general in sauropods has been challenged (Wings and Sander, 2007). Regarding point (1), a polish similar to that seen on the ‘gastroliths’ of the Sonorasaurus quarry is not expected (Wings and Sander, 2007). Regarding points (2) and (3), the Turney Ranch Formation contains abundant ‘gastroliths’ regardless of the presence of associated bone (MDD, BZF, NJ personal observation, 2014), as well as other ‘exotic’ clasts such as cobbles of Paleozoic limestone and pre-Phanerozoic reworked zircon crystals (Dickinson et al., 2009; see above/below). Regarding point (4), the ‘gastroliths’ attributed to Sonorasaurus were scattered around the quarry, and


In order to understand the lower–level relationships of Sonorasaurus,we used the character-taxon matrix from a recent cladistic analysis designed to investigate the relationships of early titanosauriforms (D’Emic, 2012). We chose to use the smaller matrix of D’Emic (2012) rather than the larger matrix of Mannion et al. (2013) because the latter employs many interrelated characters. For example, Mannion et al. (2013) separately coded the gracility of the humerus (character 42), radius (character 45), ulna (character 50), and first metacarpal (character 53). However, these are biologically related features in sauropods—sauropods with gracile forelimbs tend to have each long bone elongated, not just one segment (McIntosh, 1990). Likewise, characters 141 and 144 of Mannion et al. (2013), which code the extent and style of pneumaticity in the dorsal vertebrae, are related. Other characters are also biologi- cally related: Mannion et al. (2013) separately coded variation in the cross sectional shape of the pubic peduncle of the ilium (C57) and the iliac peduncle of the pubis (C58), but since these structures directly articulate, their shapes should not be coded independently. A similar problem exists in coding the promi- nence of the olecranon process of the ulna (C233 of Mannion et al., 2013) separately from the depth of the supracondylar fossa on the posterior face of the distal humerus (C228 of Mannion et al., 2013), which receives the olecranon process. Similarly, characters C15, 16, 17, 21, 22, 25, 26, 28, 29, 30, and 31 deal with the proportions of vertebral centra; though this feature does display variation along the column, it does not vary indepen- dently as eleven characters. An argument could be made that all of these characters are independent if they do not display iden- tical scorings, but subtle differences could be due to the vicis- situdes of preservation or the delineation of states by a given researcher; their biological relatedness indicates that they should not be split (see Gingerich et al., 2010). The effect of over- splitting these characters gives them greater weight, biasing the analysis in favor of them. A full discussion of the effects of character atomization and appropriate matrix size is beyond the scope of this paper—for recent debates of this issue, see dis- cussions surrounding pterosaurs (Nesbitt, 2011, Bennett, 2013), turtles (Gaffney and Jenkins, 2010; Joyce and Sterli, 2012), and primates (Williams et al., 2010; Gingerich et al., 2010). We made modifications to the matrix of D’Emic (2012)


for our analysis of Sonorasaurus. We added two OTUs, Sonorasaurus and Lusotitan, and we altered 15 character state scores for Europasaurus based on recently published material (Carballido and Sander, 2013; Marpmann et al., 2014). These include character 9 (1 to 0), characters 18, 30, 31, 33, 35, 36, 49, 53, 56, 57, 63 (? to 0), and characters 51 and 59 (? to 1). Three characters (Table 3) were added to the matrix ofD’Emic (2012). First, a character was added to describe the posterior elongation of middle dorsal vertebral centra and consequent anterior dis- placement of the neural arch. In most sauropods, the middle


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